This invention relates generally to the curing of articles by vulcanization. In the following description the terms curing and vulcanizing are used interchangeably and particular reference is made to a vehicle tire cured in a press or mold having an insertable bladder into which high temperature water may be introduced for shaping, curing and molding after which, cold water may be introduced for final curing. In some instances a bladder or other conventional flexible diaphragm means, may be dispensed with. Such operations using a bladder, and sometimes not using any bladder, are conventional and widespread in the tire industry. Most often used are "pot-heaters" or automatic tire shaping and curing presses such as those known by the trademark Bag-O-Matic, manufactured by the McNeil Corp., and those known by the trademark Autoform manufactured by NRM Corp., both of Akron, Ohio, inter alia. These types of automatic presses generally employ separable mold halves with center shaping and curing mechanisms utilizing bladders into which shaping, heating and cooling gases, fluids or other media are introduced for the shaping, molding and curing of tires.
Prior art systems used for controlling the vulcanization of a tire have been classified as operating in either an "elapsed time" mode or in an "elapsed cure equivalents" mode, the latter being far more desirable because the number of cure equivalents required for a cure gives a more accurate determination of the time when the precise optimum cure is effected. A "cure equivalent" is defined as one minute of curing time at a constant reference temperature, usually 280.degree. F.
The calculation of the optimum number of cure equivalents has been made in the prior art based on the Arrhenius equation in which temperatures measured within the tire reflect the effect of thermal diffusion and heat build-up. How this empirical relationship set forth in the equation is used, is taught in U.S. Pat. Nos. 3,819,915; 3,980,743; and 4,022,555 to Thomas W. Smith; and in the application of Diehr and Lutton, Ser. No. 607,463, now U.S. Pat. No. 4,344,142, at 209 USPQ 1 and decided Mar. 1, 1981 by the U.S. Supreme Court, all of which teachings rely on the preselection of a specific fixed point as being the "point of least cure" ("PLC" for brevity), so referred to because it is the critical point at which the desired optimum number of cure equivalents is to be delivered. But such a PLC is by no means fixed.
Obtaining a "perfect" cure may not be critical in some applications but it is especially desirable in aircraft tires, off-the-road (OTR) tires, and even in some relatively very much smaller articles such as V-belts. It is unnecessary to dwell, either on the severe strains to which aircraft tires are routinely subjected, or to the enormous economic losses, both in labor and material, which are incurred when an OTR weighing several thousand pounds is either unacceptably over-cured or under-cured.
The location of the point of least cure (PLC) depends upon (a) the geometry of the tire, (b) the applied boundary temperatures at each surface of the tire being molded, and (c) the thermal diffusion within the tire because of the physical characteristics of the rubber and other components of the tire. Since, even among tires of the same size, there is some variation, it will now be evident that this PLC varies in the same mold, from one tire to the next, the extent of the variation depending upon the variations from one tire to the next, and, particularly if there is any change in the temperature conditions. Since this PLC varies even when all conditions are set at those chosen, it will vary greatly when there is an "upset" in operating conditions. No prior art teaching permits the tracking of this PLC, though with enough actual measurements of temperature at various points within the tire during the curing process, this point is trackable. The self-evident impracticality of doing this is the reason why it is not done.
Stated differently, prior art teachings fix the PLC. It is at this specifically preselected PLC that the necessary determinations are made, irrespective of just how they are made. If the PLC chosen is not the precise PLC for the particular tire being cured, the determinations are nevertheless made at the preselected PLC, which is not the actual PLC, the desired number of cure equivalents are not delivered to the actual PLC, and the resulting time for vulcanizing the tire is not the precise optimum time.
It will be apparent to one skilled in the art that the PLC is calculated, and somehow located in the prior art, because it is critical that no part of the cured tire be under-cured. A relatively substantial degree of over-curing is generally not unacceptable, compared with even a small degree of under-curing. Clearly, if all parts of a tire, whether large or small, were cured for exactly the same number of cure-equivalents, no process requiring calculations would be necessary. It is therefore implicit that, while the PLC is tracked and located in the process of this invention, and while the precise number of cure equivalents desired for the PLC after it is tracked, is an input (as the goal) into the computer, some portions of the tire may be slightly overcured. Such slight overcuring in some portions of a very large tire is inevitable. However, any under-curing which is either demonstrable or unacceptable is avoided.
No prior art reference suggests a process for vulcanizing a tire based upon a determination of optimum time despite varying operating conditions from one tire to another tire of the same geometry and construction, which determination is refined sufficiently to track the PLC as it varies from tire to the next, regardless of what engineering functions are used to make such a determination.
It is well recognized that the art of temperature-sensing permits the physical measurement of temperature at any point upon, or inside, either the article to be vulcanized, or the mold, and such measurements can be, and have been made. It is not so much the measuring of temperature at any location in the system, but the reason for measuring such a temperature at specific points, which reason and specifically chosen points become crucial to the solution of the problem. As is also well recognized, the art is replete with numerous different solutions to the problem, based on as many reasons for expecting that each solution is better than, or more convenient, economical or practical than another.
The choice of temperature sensing locations is determined by the need to obtain measurement of the criteria most likely to provide the parameter desired--namely, the number of cure equivalents at a specified point. The prior art specifies this point, based on fixing its determination from desired operating conditions, and, on the assumption that no upsets due to leakage of hot water, an unexpected change in the temperature conditions in the system, or other upset, will occur. In a tire plant, it is by far better to cure a tire based on a trackable point of least cure.
In the Smith patents referred to hereinabove, a probe is inserted at the point of least cure. In the U.S. Pat. No. 4,022,555, an electrical analogy is used to determine the point of least cure for a mold of particular geometry and specific operating conditions, and it is at the point so determined that the probe is inserted. In the Diehr et al invention, a single constant (x) is used to define the geometry of a mold (which is the negative of the article), which, because it is constant, cannot reflect the effect of changes in boundary conditions. In U.S. Pat. No. 3,649,729 to Davis et al, they teach sensing the temperature of at least two boundary surfaces of a tire, one of which must be inside the tire. Davis, as in other prior art references, must necessarily choose a critical point at which the desired number of cure equivalents is to be delivered. Thereafter, his determinations lack the refinement for thermal diffusion.
Though U.S. Pat. No. 4,044,600 to Claxton et al takes into account thermal diffusion in the tire and simulates actual curing conditions with an analog resistance-capacitance network, he must predetermine the location of the point of least cure. This predetermination provides a reference input. It will be noted that they also teach that measuring the inlet temperature of the hot water to the bladder produces sufficiently accurate monitoring of the internal temperature of the tire, and failed to recognize that it is much more important to measure the temperature of the hot water after it leaves the bladder, as the exit temperature more accurately reflects what happens to the tire while it is being cured. Other references teach monitoring the temperature of the hot water for different purposes, as for example U.S. Pat. No. 2,204,531 to Erbguth who monitored the exit temperature from the bladder to ensure the operator's safety. The press could not be opened if there was a malfunction which might otherwise cause the operator to be burned.
From the profusion of teachings in the art, it will be evident that vulcanization, particularly of pneumatic tires is not easily controlled, because of the complexity of the equipment necessary for imparting the exact desired shape to the tire, and the many variables which have an all-important influence on the time-temperature factors governing the progress and completion of the vulcanization process.
These variables include residual heat from the last previous cure in the external mold, and in the bladder, the ambient air temperature at which green tires to be cured are stored, the cooling effect of the ambient surroundings on the mold and bag, the time the mold remains open before another green tire is placed in it, the time at which heating of the mold is commenced, when heat transfer fluid commences to flow into the bladder (whether before, or simultaneously with, or after placement of a green tire in the mold), and, most of all, the variation, whether programmed or not, of the quantity and temperature level at which heat is supplied to the mold after it is closed.
A change in any one of these variables can influence the progress of the vulcanization reaction in the inside of the tire, or near the outer surface, or both, so that a very complex situation is presented, which has not been easily controlled, and which has resulted in rejection of many unacceptably undercured or overcured tires, or in costly price adjustments because of premature failure or unduly rapid wear.
The best solution of the problem heretofore has been to insert a thermometric probe in the tire and connect it to a computer which opens the mold, for removal of the tire, when the time and temperature of heating in the middle of the thickness of the tire are just sufficient for optimum vulcanization. Even if such a probe does not leave a hole in a portion of the body of the tire, which it generally does, invasive monitoring of temperatures is not desirable. Such probes ar disclosed, for example, in U.S. Pat. Nos. 3,728,721; 3,980,743; and more recently in 4,115,046.
Extending the foregoing teachings of Smith, Diehr et al, Davis and Claxton et al, inter alia, it will be evident that if numerous actual measurements are made continuously within the body of a tire while it is being cured, an excellent profile of the temperatures at each of the points monitored as a function of time, would be generated, and from this information, the PLC could be tracked and located with the aid of an Arrhenius equation using the temperatures monitored. However, to accomplish the same result by sensing only two boundary conditions, regardless of where such boundary conditions are sensed, along with the ambient temperature, and without invading the body of the tire, is quite unexpected and has never been suggested. It is the implementation of the concept of tracking and locating the PLC within the body of a tire, using a microprocessor which has a relatively limited (`small`) computational capability to operate a pot heater or curing press, with which this invention is concerned.